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P & Ca precipitate in process waste

P & Ca precipitate in process waste

P & Ca precipitate in process waste


Hi All!
The plant I’m working on has got a reoccurring problem with white precipitate (slurry) forming in the process waste sump.
This precipitate is continuously blocking downstream strainers, heat exchangers, and WT plant having tremendous men-hours spent on cleaning. The chemical analysis showed P and Ca to be the main ingredients in the slurry.
The chemist who carried out the analysis suggested the use of phosphatic acid based de-scaler. The required ratio of the de-scaler is however high and due to the large waste volumes pushes the relevant environmental licencing.
Would you have any experience with mechanical extractor or a skimmer that can remove the precipitate to be dried and mechanically removed.
If of significance the waste temperature is around 80 °C / 175 F which may be promoting some salts crushing. The only source of Ca on the plant is an RO-DI water skid.

All suggestions are welcome!


RE: P & Ca precipitate in process waste

Is it possible to seperate the RO waste from the the P waste. Not sure about the total chemistry but a couple of things.
Ca has retrograde solubility in that it is less soluble at high temps. So adding it to the 80C water may be the cause of the problem. Likewise Calcium phosphate at higher e pHs is not soluble. Is there some potential to seperate these or operate at lower pH opr temperature?

"Any water can be made potable if you filter it through enough money"

RE: P & Ca precipitate in process waste

Yes, we can potentially try to split the RO waste and join it with process waste downstream from the cool down heat exchangers. Thanks!

RE: P & Ca precipitate in process waste

Boiler - you may also want to check the hardness (calcium) level of the water supply to the RO. We have an RO that is supplied with 70 gpm and rejects 15 gpm of water that is concentrated in salts. The flow ratio will tell you an approximate increase in salt concentration.

If splitting the RO rejection water past the warmer water does not solve the issue, then you may need to explore water softening upstream of the RO unit. Incidentally, if the water has too much Ca in it, you can experience Ca scaling on the RO membranes, reducing effectiveness! We have an ion-exchange resin upstream of our RO, though in our case it is used to remove chlorine from the water.

RE: P & Ca precipitate in process waste

If you can get the temperature down to a value < 120 F you'll see a huge reduction in percipitation. The solubility curve has a sharp break (reduction in solubility) at around 125-130F. At the temperature that you're operating (175F) I don't think there are any practical options for preventing this problem. I'm assuming that installing a water softener isn't a practical alternative.

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